1. Field of the Invention
This invention relates to a modulating method and a demodulating method as well as a modulating apparatus and a demodulating apparatus suitable for use, for example, for digital recording and reproduction of data onto and from a record medium.
Particularly, the modulation code used in the present invention is a variable length code, and recording data in units of m bits is converted into a recording code in units of n bits.
2. Description of the Prior Art
In a magnetic recording system, generally a signal has a differentiation frequency characteristic and suffers from deterioration in a high frequency band. This arises from a loss at a head gap, a loss by a space between a head and a record medium, a loss originating in thickness of a record medium, a low frequency band loss at a rotary transformer and so forth. Further, noise such as crosstalk noise from an adjacent track, noise from a record medium and overwrite noise could make a cause of a random error. Meanwhile, in an optical recording system, the output decreases substantially linearly toward an optical cutoff frequency (2NA/.lambda.) which depends upon the wavelength of a light source and the numerical aperture of the lens. In order to record and reproduce data accurately irrespective of such losses and noise, digital information should be recorded onto a record medium after it is modulated so that it may be compatible with a recording and reproduction system, which allows a greater amount of information to be recorded stably. To this end, channel coding of data (recording signal) is performed in accordance with a predetermined rule.
One of such channel coding methods is block coding. The block coding involves dividing a data sequence into blocks of m.times.i bits and converting each data word into a channel code of n.times.i channel bits in accordance with a predetermined coding rule. When i=1, the channel code is a fixed length code, but when i is greater than 1 and the binding length r is greater than 1, the channel code is a variable length code. The block code is also called (d, k;m, n;r) code. Here, d is a minimum run number of 0 while k is a maximum run number of 0.
The block coding is disclosed in Japanese Patent Laid-Open Application No. Heisei 01-221919, laid open on Sep. 5, 1989, under the title of "Variable Length Code Converting Method". According to the variable length code converting method, the run numbers of 0 at the beginning and last ends of a code word are restricted and a uniformly decodable code word is selected. Consequently, a code having a minimum magnetization reversal distance (Tmin) which is 1.33 times that of a conventional (2, 7) RLL code and is twice that of the MFM.
In particular, according to the variable length code converting method, the minimum data length m is m=2, the minimum code word length n is n=6, the code word length number rmax is rmax=4, and the run number of "0" of a binary code word train of continuous code words after conversion is restricted equal to or greater than 5 but equal to or smaller than 16 (d, k). The requirements for such variable length code to be used as a code word include, in addition to three requirements that the (d, k) restriction is satisfied within one code word, that a boundary of a code word can be discriminated correctly and that a code word can be decoded uniformly, an additional requirement that code words are allocated preferentially from that one which includes a greatest number of "1s" so that the average magnetization reversal distance may be minimized.
Similar block coding techniques are disclosed in Japanese Patent Laid-Open Applications Nos. Heisei 01-221920, Heisei 01-221921, Heisei 01-221922, Heisei 01-221923, and Heisei 01-221925, all laid open on Sep. 5, 1989.
The variable length code disclosed in the prior art documents is disadvantageous in that, when compared with the variable length code employed in the present invention, the detecting window width Tw is narrower and the detecting efficiency upon decoding is lower.
Another block coding technique is disclosed in Japanese Patent Laid-Open Application No. Showa 59-160357, laid open on Mar. 3, 1983 under the title of "Coding and Decoding Methods for Binary Data", wherein binary data of 2 bits is converted into a conversion code of 5 bits to enhance, while making the most of a magnetization reversal distance of the HDM-3, the detecting window width to 0.4 T to allow recording and reproduction of data in a high density.
More particularly, according to the block coding technique, binary data are outputted successively in synchronism with a clock signal to terminals for parallel outputs of a serial/parallel register of 10 bits. A coder receives, at input terminals thereof, the data from the output terminals of the serial/parallel register, generates a 5-bit conversion code in accordance with a coding algorithm, and outputs it to 5-bit output terminals. The 5-bit conversion code is inputted to a 5-bit serial/parallel resistor, from which a conversion code train is outputted to an output terminal of it.
The coding method, however, does not involve a variable length code but involves a fixed length code, and employs a code resembling the variable length code employed in the present invention. However, the algorithm of the generating method of the code is complicated, and accordingly, also the hardware configuration is complicated.
A further block coding technique is disclosed in "Proposal of (3, 19;4, 9;3) Code" Collection of Theses of Spring National Conference of Electronic Information Communication Society, 1989, which describes a (3, 19;4, 9;3) code as a concrete example of a (3, k) variable length code. The code is superior in minimum magnetization reversal distance Tmin to the (2, 7) RLL code and the 2/3 code and is superior in Tmin.times.Tw (minimum magnetization reversal distance.times.detecting window width) to the (2, 7) RLL code. It is to be noted that characteristic evaluation of the case wherein the code is estimated to be used for a magneto-optic system is disclosed in Television Society Bulletin, Vol. 44, No. 10, pp.1369-1375, 1990.
The conventional codes described above have a subject in that the value of the minimum run number d is low (d=2) and the minimum reversal distance Tmin is small. Generally, record media, particularly optical record media, are remarkable in deterioration of a reproduction output in a high frequency band, and in order to achieve high density recording, it is desired to increase the value of d and increase the minimum reversal distance Tmin.
On the other hand, if the value of d is increased, then the value of m is increased and also the value of the binding length r is increased. As a result, the conventional codes have another subject in that a conversion table with which digital data is to be converted into a variable length code is increased in size.
Further, in the case of a variable length code, since a conversion code is different among recording data, it may be exceed the length of a block. In this situation, when it is tried to demodulate a code completely within a block, unless some measure is applied to a modulation code, the conversion code exceeding the length of a block may make a cause of failure in decoding or a cause of an error in result of demodulation. Further, when demodulation is performed across two blocks, if it is tried to begin demodulation with an intermediate portion of a block, failure in demodulation may take place since there is no code of a preceding block.